It has been proposed that cyclic AMP plays an important role in control of cell proliferation of mammalian cells. Cyclic AMP stimulates steroidogenesis and inhibits the growth of mouse adrenal tumor cells (Y-l). We have isolated cAMP-resistant mutant cells from Y-l cells. The properties, such as ACTH-receptor, adenyl cyclase, cyclic AMP receptor and cyclic AMP-dependent protein kinase activities, will be studied and compared with Y-l cells. Both Y-l and cyclic AMP-resistant cells require serum for growth in culture. Substances in serum responsible for growth of both cells will elucidated. Comparison of these properties between Y-l and cyclic AMP-resistant mutant will be useful in understanding the role and mechanism of cyclic AMP action.